An ink jet recording method is a printing method of ejecting droplets of an ink composition and attaching them to a recording medium such as paper, thereby performing printing. This ink jet recording method is characterized by that images having high resolution and high quality can be printed at high speed. The ink composition used in the ink jet recording method is generally one mainly comprising an aqueous solvent and containing a coloring material component and a wetting agent such as glycerin for the purpose of preventing clogging.
When printing is performed on paper or cloth of a type that is hard for the aqueous ink composition to penetrate thereinto, or a material such as metal or plastic into which the aqueous ink composition does not penetrate, for example, a recording medium such as a plate or a film made of a resin such as a phenol resin, a melamine resin, a vinyl chloride resin, an acrylic resin or a polycarbonate resin, the ink composition is required to contain a component which can allow the coloring material to be stably fixed to the recording medium. In particular, when printing is performed on a printed-circuit board, a color filter of a liquid-crystal display, or the like, quick drying properties and chemical resistance are demanded for the ink composition.
To such a demand, an ink composition comprising a component polymerizable by ultraviolet irradiation has hitherto been proposed (see, for example, patent document 1). Further, an ultraviolet curing ink composition comprising a coloring material, an ultraviolet curing agent, photopolymerization initiator and the like has been proposed (see, for example, patent document 2). It is disclosed that according to these ink compositions and ink jet recording methods, blurring of the ink composition on the recording medium can be prevented to improve image quality.
In the ink jet recording method using the ink composition comprising the component polymerizable by ultraviolet irradiation as described above, an ultraviolet lay is irradiated after the ink composition has been attached to the recording medium. Thereupon, the photopolymerization initiator in the ink composition forms radicals, thereby initiating polymerization of an oligomer and a monomer to cause curing. Accordingly, the coloring material in the ink composition is firmly fixed onto the recording medium. It is considered that this firm fixing enables to realize printing which is high in film strength, solvent resistance and color density and has decreased blurring and unevenness.
It has been known that monomers used in the conventional ultraviolet curing inks generally have the tendency that one having high curability is high in the primary irritation index (PII), whereas one having low PII is low also in curability. In the conventional ultraviolet curing ink compositions, materials having a PII of 5 or more are used as raw materials in many cases in order to enhance the rate of polymerization, and attention is required for handling thereof. An equipment burden such as the use of a local exhaust system is necessary upon use, and this has not been favorable.
Further, the rate of polymerization by ultraviolet curing generally tends to increase as the molecular weight of a monomer used decreases.
From the relationship between the high and low of the above-mentioned PII and the high and low of curability (rate) and the relationship between the high and low of the rate of ultraviolet curing polymerization and the high and low of the molecular weight of the monomer used, (in the case of similar structures), the lower the molecular weight of the monomer used is, the higher the rate of polymerization is, but also the higher the PII is. The higher the molecular weight of the monomer used is, the lower the PII is, but also the lower the rate of polymerization is, and further, the viscosity also becomes high (for example, in order to decrease the PII, a methacrylate monomer is more effective than an acrylate monomer, but it decreases the rate of polymerization and raises the viscosity).
Furthermore, it is also advantageous to use a multifunctional monomer as a measure for decreasing the PII and enhancing the rate of polymerization, but it raises the viscosity.
In order to decrease the viscosity of a high-viscosity ink composition, it is forced to use an organic solvent (VOC) which is high in volatility (evaporates at ordinary temperature) in a large amount. This has not been favorable also from the viewpoint of the environment.
Patent Document 1: JP 3-216379 A
Patent Document 2: U.S. Pat. No. 5,623,001
As described above, various attempts have been made to the demands of decreasing the PII, enhancing the rate of polymerization and decreasing the viscosity. However, the development of an ink composition which meets all the high rate of polymerization, low PII value and low viscosity has not been achieved.